Anaphylotoxins are known to generate inflammation. In part, this may be through their ability to activate mast cells. Both C3a and C5a were thus found to be potent chemotaxins for the human mast cell line HMC-1. Chemotaxis of mast cells to both C3a and C5a was blocked by pertussis toxin, suggesting that Gi-coupled receptors are involved in signal transduction. C3a and C5a also induced transient pertussis toxin-inhibitable increases in [Ca2+]i that could be homologously but not heterologously desensitized. Chemotaxis of mast cells to C3a exceeds chemotaxis to SCF and to TGF-Beta, making C3a the most effective human mast cell chemotaxin identified to date. Its potency as a mast cell chemoattractant is also 100-1000-fold greater than for all of its previously described cellular actions. These studies demonstrate that C3a and C5a initiate signal transduction in mast cells which results in directed migration of mast cells and may help explain the rapid accumulation of these cells at sites of inflammation where complement activation is taking place. To determine if human mast cells express NGF receptors, both the TrkA tyrosine receptor and the low affinity NGF receptor (LNGFR) were searched for in a human mast cell line (HMC-1) and in human mast cells cultured in the presence of stem cell factor. Both HMC-1 and cultured human mast cells were found to express TrkA. TrkA protein was demonstrated by Western blot analysis of HMC-1 lysates. Using flow cytometry with mast cell tryptase as a mast cell marker, both HMC-1 cells and cultured human mast cells were shown to co-express tryptase and TrkA. Treatment of mast cells with NGF resulted in phosphorylation of TrkA on tyrosine residues as detected by immunoblotting with an antiphosphotyrosine antibody. NGF induced the immediate early gene c-fos in HMC-1 cells. HMC-1 cells and cultured human mast cells were also found to express NGF mRNA, and conditioned medium from HMC-1 cells stimulated neurite outgrowth from chicken embryonic sensory ganglia in culture. This effect was blocked by anti-NGF. Thus, mast cells express functional TrkA and synthesize NGF, suggesting a mechanism by which NGF may act as an autocrine factor for human mast cells, and by which mast cells and nerves may interact.